Melting alloy and bimetallic overload relays are designed to approximate the _____ generated in the motor?

• A. Voltage
• B. Current
• C. Heat
• D. Sound

Answer: (C)

The key idea is that these overload devices protect a motor by sensing the heat that would build up inside it during overcurrent conditions. When a motor draws more current than its rating, the electrical energy lost as heat in the windings (I^2R losses) rises, and this heat is what can cause insulation damage if not limited. Melting alloy and bimetallic overload relays use a heat-sensitive element—either a melting alloy or a bi-metal strip—that responds to temperature increases. As the motor heats up from excess current, the element reaches its trip temperature and causes the relay to open, disconnecting power before damage occurs. This approach focuses on the heat generated, which is the actual factor that leads to motor failure, rather than voltage levels or sound. While current influences heat, the protection is fundamentally a heat-based mechanism tied to how much and how long the motor is overloaded.